(a) Field of the Invention
This invention relates to a geothermal heat pump for residential or commercial use falling under the general category of what is known in the industry as a "direct expansion" heat pump, (hereinafter referred to as a "DX" heat pump).
(b) Description of the Prior Art
Conventional technology concerned with heat pumps relies upon the transfer of heat from the ground by means of a secondary working fluid, e.g., water, which is pumped to the geothermal unit located in the heated structure. The conventional heat pump has its own internal heat exchanger which extracts the heat (heating mode) from this water, which is then pumped back to the earth to be reheated.
Such geothermal heat exchange is an efficient and effective way of achieving heat exchange in heating and air conditioning systems, and especially heat pump type systems. Since the ground temperature is relative constant at about 48.degree. F. at a depth below the frost line, the available heat is constant.
"DX" systems similarly use a ground coil system. However the working fluid is a refrigerant and the copper ground loop is the primary heat exchanger. A problem which has been associated with such systems is the means and manner in which the heat exchange coils, or outdoor coils, are placed into the ground to achieve geothermal heat exchange.
If the geothermal outdoor coils are placed into the ground in a vertical fashion, installation may be easily accomplished by drilling or boring holes into the ground, into which the vertical geothermal outdoor coils may be placed. The coils may quickly and easily be placed into the ground to a depth which is sufficient to overcome ground freezing problems associated with colder climates.
Heretofore, one reason why placing coils into the ground in a vertical fashion has not been workable was due to the fact that, when sufficient refrigerant was placed into the system to achieve maximum efficiency in both the heating and cooling cycles, the refrigerant, as it condensed in the ground coils, caused a liquid refrigerant build-up. The compressor was unable properly to move the refrigerant through the system when the liquid refrigerant settled within the ground coils, making the system unworkable. Damage to the compressor could occur when the compressor forced liquid refrigerant into the intake of the compressor, since compressors for such systems are designed for receiving and compressing gases.
Such problem associated with vertical outdoor geothermal coils was attempted to be solved by placing the coils into the ground in a horizontal fashion. Placing the coils into the ground in a horizontal fashion partially alleviated the problem of liquid refrigerant build-up. However, this technique required a vast amount of available ground to achieve the proper heat exchange, as well as the excavation of sufficient land to place enough ground coils to achieve sufficient heat exchange. In colder climates, this excavation must also be to a sufficient depth to place the coils for proper heat exchange. In short, placing the geothermal coils in a horizontal fashion was more difficult, expensive, and required much more available ground than does placing of the coils into vertical holes.
It has also been common practice to provide heat pump systems which included means for reversing the direction of flow of refrigerant from the compressor through two heat exchangers so that the functions of the heat exchangers were reversed. Thus, one of the heat exchangers was adapted to heat or cool the air of a room, for example, according to the direction of flow of refrigerant. When the direction of flow of refrigerant was reversed, it was necessary to alter the degree of restriction between the two heat exchangers to provide for proper operation of the system in the alternative function. Altering the degree of restriction relative to the direction of flow of refrigerant was necessary since a system optimized for cooling generally had insufficient restriction to provide optimum performance when operated to supply heat. That is, in a system optimized for cooling, the compressor normally circulated refrigerant through the evaporator faster than the surface could evaporate the refrigerant when the system was operated in the heating cycle. The compressor, in the heating cycle, then pumped unevaporated refrigerant and the system efficiency was low. To overcome this problem, variable restriction systems, using a "TX" or thermostatic expansion valve, have been employed in such heat pump systems. In many instances, two restrictors have been provided together with parallel valve systems, each of which was adapted to function in accordance with the direction of refrigerant flow.
A direct earth coupled heat pump was one that had its refrigerant evaporator/condenser in direct thermal contact with the earth from which heat was either extracted from in the heating mode or is introduced to in the cooling mode of operation. Many attempts have been made in the past to develop successful direct coupled heat pumps for residential and commercial uses. These attempts have failed adequately to meet a number of requirements associated with an economically and functionally viable system. Some of the shortcomings included: inadequate oil return to the compressor primarily in the heating mode; inadequate evaporator length and spacing for properly extracting heat from the earth resulting in low capacity and low efficiency of the systems; lack of a proper means to store additional refrigerant required during the cooling operation, but not needed during the heating mode; lack of volume control of the compressor for providing the necessary increase in displacement during the heating operation over that displacement needed for the cooling operation. This lack of displacement control resulted in insufficient heating capacity during the coldest weather.
U.S. Pat. No. 4,920,757 patented May 1, 1990 by J. Gazes et al attempted to provide a solution to overcome the problems associated with heat exchange with the outside air, water and geothermal means which have been employed for heat exchange. The patented solution was the provision of a geothermal heating and air conditioning system which included a compressor for compressing a refrigerant, and one or more indoor coils for heat exchange between the refrigerant and inside air. One or more outdoor coils were placed vertically below ground for heat exchange between the refrigerant and earth surrounding the outdoor coils. A condenser/receiver allowed the refrigerant to pass through the condenser/receiver in either direction, and was positioned between the indoor coil and the ground coils, allowing for heat exchange so as to control the state and temperature of the refrigerant. The condenser/receiver had sufficient volume capacity to accumulate liquid refrigerant to prevent liquid refrigerant from entering the compressor. A portion of the refrigerant travelling between the indoor coils and the outdoor coils was diverted to an inlet side of the compressor. A valve controlled a flow of refrigerant in response to refrigerant pressure on the inlet side of the compressor.
U.S. Pat. No. 5,038,580 patented Aug. 13, 1991 by D. P. Hart attempted to provide a solution to problems associated with heat pumps having direct earth coupled heat exchangers. The patented improvement provided a heat pump system having a sub-surface heat exchanger which used a heat exchanging fluid existing in gaseous and liquid form. The heat pump system included a compressor for compressing the heat exchanging fluid. A four-way reversing valve for directing the flow of the heat exchanging fluid was functionally connected to the compressor. An indoor heat exchange coil, functionally connected to the four-way reversing valve, was provided for transferring heat to or from the interior of a building. An accumulator, also functionally connected to the four-way reversing valve, was provided for trapping and storing liquids within the apparatus. A plurality of sub-surface tapered heat exchanger tubes were functionally connected to the four-way reversing valve. A bi-directional balanced expansion valve was functionally connected to the sub-surface heat exchanger. A receiver was provided for storing excess fluid functionally connected to the expansion valve and to the indoor heat exchange coil.
U.S. Pat. No. 4,327,560 patented May 4, 1982 by H. I. Leon et al, provided an earth-embedded heat pump system. That patent provided a system for transferring heat from the earth to a conditioned space wherein the heat transfer fluid was circulated in a substantially closed loop through an earth coil. Sections of the coil were jacketed with one or more hermetically-sealed enclosures which were adapted to be charged with a composition which, upon crystallization, released latent heat. The transition temperature at which the latent heat was released and the rate at which the latent heat was released to the heat transfer fluid was said to stabilize its temperature within a range which provided for an inlet condition to a water-to-air heat pump which was favourable to a high heat output from the heat pump system per unit energy input.
In U.S. Pat. No. 4,437,583 patented Mar. 8, 1983 by J. E. Downy, Jr., a complete system was disclosed for providing space heating and cooling, for an indoor living space which brought the heat from the geothermal storage capacity, (the earth), to the indoor living space during the heating season and extracted heat from the indoor living space and absorbed the heat into storage during the cooling season. In the patented system, a relatively massive thermal storage unit was interposed between the geothermal storage capacity, (the earth), and the indoor living space environment which was to be heated or cooled. A working fluid, e.g., water, was circulated between the massive thermal storage unit, and the geothermal storage capacity, (the earth). A heat pump was operatively connected between the massive thermal storage unit and the indoor living space environment. In addition, the working fluid, was circulated between heat exchangers, one of which was located in the massive thermal storage unit in contact with the working fluid therein and the second of which was located in the indoor living space environment.
U.S. Pat. No. 4,445,343 patented May 1, 1984 by W. J. McCarty provided a heat pump system comprising a compressor and two heat exchangers connected in a refrigerating circuit. Such refrigerating circuit was provided with refrigerant flow restricting means between the heat exchangers. Such restricting means imported relatively high restriction to the flow of refrigerant between the heat exchangers in one direction and a relatively lower restriction to the flow of refrigerant between the heat exchangers in the opposite direction.
U.S. Pat. No. 4,646,538 patented Mar. 3, 1987 by A. J. Blackshaw et al provided a heat pump system which had the capability of both space heating and cooling and which used the minimum number of components while at the same time permitting any two heat exchangers in the system to be used without involving the other heat exchanger so that any heat exchanger not being used in a particular mode could be bypassed. Further, those portions of the system not being utilized in any mode remained connected to the suction side of the compressor to depressurize that portion of the system. Liquid traps prevented the undesired build-up of refrigerant in that portion of the system not being currently used. The system design was said to permit the various operational modes by using only one additional externally controlled valve over that associated with a heat pump system used only to space heat and cool with the rest of the additional components used to interconnect the system being operated without any external control force.
The patented apparatus included a refrigerant pressurizing means whose high pressure outlet was connected to the input of a three-way valve. One output of the three-way valve was connected to the common input of a four-way valve. The common output of the four-way valve was connected to the suction side of the refrigerant pressurizing means. One of the reversible outlet ports on the four-way valve was connected to a space heat exchanger while the other reversible outlet port on the four-way valve was connected to a source heat exchanger. The opposite sides of the space and source heat exchangers were connected to each other through a reversible expansion device. The other output of the three-way valve was connected to an alternate heat exchanger. The other side of the alternate heat exchanger was connected to an alternate expansion device. The other side of the alternate expansion device was connected to the common point between the reversible expansion device and the space heat exchanger through a check valve allowing refrigerant to flow from the alternate heat exchanger to the space heat exchanger through a check valve. The other side of the alternate expansion device was also connected to the common point between the reversible expansion device and the source heat exchanger so that refrigerant could flow from the alternate heat exchanger to the source heat exchanger through a check valve.
As is evident from the above discussion, the general refrigeration cycle of a "DX" heat pump is similar to a conventional water-to-air or water-to-water heat pump in that it includes a compressor, an expansion device, a reversing valve, and a refrigerant-to-air heat exchanger. The unit functions as both a heating and cooling device and also generates domestic hot water. The "DX" heat pumps differ from conventional geothermal liquid source heat pumps in that the heat exchanger which transfers heat to and from the earth is an external part of the unit and is embedded directly in the earth in either a horizontal or vertical configuration. A "DX" machine takes the heat exchanger directly to the source of heat (the earth) while a conventional unit with a self-contained heat exchanger relies on having a fluid containing heat pumped to it for extraction. The heat pump and exchanger comprise one integral unit.